Double containment pipelines are in common use for transporting contaminants and toxins underground. They are commonly used as underground gasoline transporting pipelines. A double containment pipeline section is in essence a pipeline section within a pipeline section. An inner or primary pipeline section is the primary carrier of the fluids while the outer or secondary pipeline section is used to contain any leakage from the primary pipeline section. An annulus or interstitial space is formed between the primary and secondary pipeline sections. Typically, double containment pipeline sections are formed by slipping a secondary pipeline section over the primary pipeline section. The secondary pipeline section is typically the next largest size after the primary pipeline section.
Slipping one pipeline section over the other is a cumbersome, time consuming process because the primary pipeline section is formed separately from the secondary pipeline section and then the two are put together. Moreover, this approach is not conducive to forming double containment pipeline sections having curvatures such as double containment fittings. Sometimes the primary pipeline section is impact damaged before or during the process of being slipped into the secondary pipeline section. If the primary pipeline section is made from a composite material, such damage may not be capable of being detected by the human eye. As a result, double containment pipeline sections formed using this method may be undetectably damaged from the onset.
To detect leakage of the primary pipeline section, a sump is generally placed at the lowest point of the pipeline. The sump is typically connected to a lower portion of the annulus formed between the primary and secondary pipeline sections. Any fluid leaked from the primary pipeline section will travel in the annulus and will collect in the sump. The sump is periodically monitored to determine if there has been leakage. A sensor may be placed in the sump for detecting the accumulation of fluids. A disadvantage with this leak monitoring approach is that it is not instantaneous, i.e., a leak from the primary pipeline section that is located far enough upstream from the sump would not be detected until the leaked fluid has traveled down the pipeline and into the sump.
During installation and initial inspection of double containment pipeline sections, leakage may be detected by visual inspection of the primary pipeline section. To accomplish this type of inspection, the secondary pipeline section is telescoped away from the primary pipeline section. This is also a time consuming, costly process.
Furthermore, the use of a continuous leak detection monitoring system that places a fluid, such as brine, in the annulus (or interstitial space) has been impractical and costly for double containment pipeline. In such monitoring systems, the interstitial space is filled with a fluid substance, such as brine, and connected to a fluid reservoir where the brine levels are monitored. This allows for continuous monitoring of leaks in both the primary and secondary pipeline sections because a leak in either would change the volume of fluid in the interstitial space and, thus, the fluid reservoir. The use of fluid for continuous monitoring has been practical in double-walled tanks, but not in pipeline systems. Unlike underground storage tanks that are assembled in factories, double containment pipeline systems must be assembled in the field, which makes filling the interstitial space with fluid difficult.
Additionally, known double containment pipeline sections having one pipeline section mounted over another pipeline section typically have a large interstitial space, which requires large volumes of fluid.
Another type of contained pipeline system that has been used for conveying fuel from tanks to dispensers is extruded, flexible hosing. Typically, in such contained systems the flexible hosing has a “coaxial” construction, wherein the primary layer and secondary layer are in close proximity. Although the coaxial construction lowers the volume of the interstitial space and the amount of brine required to fill the interstitial space, the material properties of flexible hoses are not stiff enough to resist the significant changes in volume that occur in the primary hose when it is under internal pressure (i.e. the diameter expands). This change in volume is a reason why a continuous monitoring leak detection system has been practical for underground storage tanks, but not for flexible hosing.
Underground storage tanks are typically filled with fuel and remain at atmospheric pressure. Therefore, no pressure differential exists between the tank interior and the interstitial space between the walls. In contrast, pipeline used for dispensing or transporting fuel from the underground storage tanks are typically operated with internal pressure on the interior pipeline section. The internal pressure changes, for example, when a submersible turbine pump (STP) located at the tank is turned on by a consumer at a fuel dispenser and when a nozzle at the dispenser is turned off when a full tank of gas is detected. The pressure change at the starting of the pump is typically approximately 30 psi, and the spike in pressure at the nozzle shut-off can be over 200 psi. Thus, the fuel dispensing pipeline sections must be able to withstand tremendous pressure. With respect to the flexible hosing, the primary layer cannot sufficiently withstand the high pressure changes without expanding under pressure, thus affecting the volume of the interstitial space. Therefore, although the coaxial construction of the flexible hosing has a low volume interstitial space, continuous monitoring of the interstitial space has been impractical and unreliable.
Furthermore, attempts to use continuous monitoring in double containment pipeline sections or coaxial flexible hoses have only been able to monitor the pipeline sections themselves and not the fittings that connect the pipeline sections. Rather, in such systems, where the pipeline sections are joined together by a fitting (which includes elbows, tees, etc.), the interstitial spaces of the two pipeline sections are not continuously joined and are not in fluid communication with each other. Instead, the fittings only maintain a continuous connection between the primary pipeline sections, and the interstitial spaces of the two pipeline sections are separately connected by some form of bypass hose, jumper device, or boot device. Therefore, known systems fail to maintain a continuous leak detection system for the entire system, including at the fittings.
Accordingly, there exists a need for a contained pipeline system with a double containment pipeline section having a coaxial construction that exhibits high strength and has a low volume interstitial space. Moreover, there exists a need for a system where the fittings that connect the various pipeline sections maintain a continuous connection between the primary pipeline sections and the secondary pipeline sections. Use of such pipeline sections and pipeline fittings would allow for a contained pipeline system where brine could be filled in the interstitial space to provide a continuous leak detection monitoring system of both the primary and secondary pipeline sections, as well as at the pipeline fittings.